The present invention relates to a method of and an arrangement for calibrating a photographic color copying apparatus in general, and more particularly for calibrating a copying apparatus in which photographic copying material is exposed to light in the three basic colors and in which the exposure is controlled by a photoelectric color exposure control device. Even more particularly, the present invention is concerned with calibration which is based on the evaluation of developed test images on the copying material.
One conventional approach in this respect was to produce a test image on the copying material from a calibrating original having a middle density, by resorting to the photoelectric color exposure control device. The developed image was then evaluated and, based on experience, there were made corrective adjustments of the photoelectric color exposure control device in such a manner that, presumably, the necessary changes in the exposures would occur resulting in a satisfactory quality of the copy. Thereafter, the copying operation was repeated with these corrections from the same calibrating original and, should these corrections indeed have brought about the desired result or at least a marked improvement of the quality of the test image, these corrective values were then stored in the corresponding storing potentiometers as changes in the sensitivity of the photoelectric color exposure control device. This approach was used, for instance, in German Pat. No. 2,246,544.
An important drawback of this approach was that it required the performance of a significant number of steps, the results of which depended on the subjective evaluation of the copied image and on the subjective determination of the needed corrective measures. As a result of this, in many instances, it was necessary to repeat the above-mentioned steps several times before the required adjustment of the photoelectric color exposure control device was arrived at.